text:
Are pilots allowed to let passengers fly the plane?
Is a private pilot license holder in the USA who is giving a scenic tour with friends allowed to let them fly the plane? Are there any restrictions on when this can be done?
In my mind, this could be done safely while at altitude and the pilot could let them do a few shallow turns.  They wouldn't want them doing anything complex and would want them to instantly let go if asked.

Related question: Are there “Introductory flights” available to see if I want to pursue training?

About ten years ago, on a regional carrier in Canada, during a commercial flight (small regional jet) where she was the only passenger, my wife got offered to pilot. She did (resulting in flight going thousands of feet higher/lower). Got sent back out of the cabin as they approached destination. Apart from the following inappropriate suggestions by the pilot (which I won't detail), how big of a deal is it regarding regulations/safety ?

@Jeffrey I'd call it a big deal, especially post-9/11, to even allow a passenger into the cockpit at all. Not sure about Canada, but in most countries doing so is now illegal under aviation law. As to flight safety, a few thousand feet of deviation is a lot, depending on where and when of course (other traffic, height over terrain, etc. etc.). a few thousand feet can put you in a mountainside, or in the path of oncoming traffic, quite easily.

For a non-commercial flight under Part 91 rules there are no issues I'm aware of with letting a non-rated passenger handle the controls - and it's done quite frequently (I can't think of any pilots I know offhand who haven't had someone else take the controls for one reason or another).
The rated pilot is still pilot-in-command of the flight and ultimately responsible for everything that happens, so they need to be relatively certain their friend will let go of the controls if told to do so, and they need to maintain situational awareness (traffic scan, airspace, etc. -- if the friend blunders into Class B airspace it's going to land in the pilot's lap) and be ready to take over if the friend starts doing something that could jeopardize the safety of the flight.

There are some common-sense limits here: something insane like have two non-rated friends at the controls the rated pilot being PIC from the back seat giving them instructions over the intercom would be a no-go.
Similarly while this sort of thing is a total non-event in an aircraft with typical dual controls you would probably want to have a higher level of trust in your friend if you're flying something with a throw-over yoke.

are you sure such things wouldn't fall under flight instruction and therefore require the PIC to have an instructor rating?

@jwenting Well, if you aren't a CFI, you clearly can't give instruction.  ;-)  (I completely agree with voretaq7 on this one!)

@Lnafziger you can't give instruction, but you can let someone else take a whirl at holding the controls for a few minutes (technically, maybe not legally, that's my question). And you can give instructions, you're just not legally allowed to do so as part of flight instruction so the other person can't log the time.

jwenting, it is legal, at least for a Part 91 flight.  I can't point to a FAR that says it is legal, but I also can't point to a FAR that says it is illegal.  So it is legal.  I know a retired USN F-14 pilot who does not have a pp-asel certificate. (He does hold a PP-AMEL cert.) It is therefore illegal for him to pilot a C-172, crazy as that is.  Having him fly right seat with a legal PIC provides quite a demonstration of airmanship!

@jwenting For it to fall under "instruction" you would have to be endorsing their logbook (for which you would need the appropriate instructor rating). In theory you could certainly teach your friend to fly (for all practical purposes) without holding an instructor rating, but that wouldn't count for anything as far as the FAA is concerned (your friend couldn't log the time as "dual received", they couldn't solo, you couldn't endorse them for the written or checkride, etc.) -- it could shave countless hours off their time with a CFI though :-)

Could you provide a reference that it's not legal under commercial flight?

@raptortech97 I can't say for certain whether or not it's legal for commercial flights - that's a very broad category of operations so I only answered for Part 91 non-commercial ops since that was the question asked. It may be permissible for Part 91 commercial ops too (I can't think of a regulation that prohibits it). I know FAA Part 121 operators (US scheduled airlines) are bound by 49CFR 1544.237 and possibly other requirements in 49CFR Chapter XII, so the legal web gets pretty tangled.

@raptortech97 See my answer on this question for the references showing that it is not legal for commercial flights.

AOPA has an article under Pilot Resources that specifically covers this situation called Logging Pilot-in-Command (PIC) Time (emphasis added by me):

Unlike driving cars, the PIC may allow anyone, including a non-pilot,
  a pilot who may not legally act as pilot in command, or another fully
  qualified pilot fly the airplane, or be "sole manipulator of the
  controls" during the flight. The PIC is not required to sit in the
  left pilot seat. Regardless of where the PIC is sitting in the
  airplane or who is manipulating the controls, the PIC is ultimately
  responsible and accountable for the safety and operation of the
  flight.

If you are the PIC letting someone else fly, remember (like they say above) that you are still responsible for everything that they do, so make sure that you never put yourself in a situation where you are not able to maintain (or regain, if necessary) control of the aircraft.
I would recommend that you carefully brief the passenger about not fighting you on the controls and to immediately let go if you tell them to.  Do this before you let them fly and be clear that it is about flight safety.   Most of them will be listening carefully once you say that.  :-)
I have done this plenty of times, and another thing you can do (especially when at lower altitudes) is to keep your hands and feet on the controls at all times so that you can stop them from doing anything too out there.

Now that being said, air carrier regulations specifically prohibit pilots from letting passengers fly so the above would only apply to private flights:

§135.115   Manipulation of controls.
No pilot in command may allow any person to manipulate the flight
  controls of an aircraft during flight conducted under this part, nor
  may any person manipulate the controls during such flight unless that
  person is—
(a) A pilot employed by the certificate holder and qualified in the
  aircraft; or
(b) An authorized safety representative of the Administrator who has
  the permission of the pilot in command, is qualified in the aircraft,
  and is checking flight operations.
§121.545   Manipulation of controls.
No pilot in command may allow any person to manipulate the controls of
  an aircraft during flight nor may any person manipulate the controls
  during flight unless that person is—
(a) A qualified pilot of the certificate holder operating that
  aircraft.
(b) An authorized pilot safety representative of the Administrator or
  of the National Transportation Safety Board who has the permission of
  the pilot in command, is qualified in the aircraft, and is checking
  flight operations; or
(c) A pilot of another certificate holder who has the permission of
  the pilot in command, is qualified in the aircraft, and is authorized
  by the certificate holder operating the aircraft.


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How does ATC handle arriving/departing runways that cross?
Forgive my total lack of aviation knowledge. I just flew out of SFO and was fascinated by the fact that pairs of planes alternated between departing on 28L + 28R (simultaneously) and arriving on 19L/R.
My question is this: What amount of micromanagement from ATC exists to keep taking-off planes on schedule and not running into the landing planes, especially in cases where runways cross? How much information does the tower give to the taxiing planes?
Thanks!

To paraphrase a much older comment about how some piece of technology works:  "Very well, thank you".  Alternately, the same way one would get to Carnegie Hall ("Practice, Practice, Practice").

Chapter 9 - Departure Procedures and Separation of the FAA Order titled "Air Traffic Control" covers the procedures that are used for aircraft departures, including those cases where they have intersecting runways.
There are numerous procedures that they follow that are specific to intersecting runways, including:

3-9-4 - LINE UP AND WAIT (LUAW)

If the tower has an airplane Line Up And Wait (taxi onto the active runway but not actually takeoff) because of another airplane that is landing or taking off on an intersecting runway, they must inform them of the other airplane and why they are not taking off.

3-9-7 - WAKE TURBULENCE SEPARATION FOR INTERSECTION DEPARTURES
3-9-8 - INTERSECTING RUNWAY SEPARATION

This probably covers most of what you are asking about, and has some great pictures showing when they are allowed to clear another airplane for takeoff in different situations.  For instance, they can't clear an airplane for takeoff with another airplane landing on an intersecting runway until after they have passed their runway.

Note that there are similar procedures in Chapter 10 - Arrival Procedures and Separation
In short, there are lots of extra procedures that must be followed when using intersecting runways simultaneously, but they also get a lot of operational benefits when they can do it so are able to get a lot more airplanes on and off the ground in the same amount of time.

Another tool used (in the USA) would be Land And Hold Short Operations (LAHSO)

As mentioned, there are lots of procedures that can be used. The one I am most familiar with (from a GA standpoint) is Land And Hold Short Operation (LAHSO). In this case, ATC may sequence you in on a runway and then ask you if you can preform a LAHSO (this happened to me last week actually).
The basic procedure is to land and stop the aircraft before the runway intersection, so that the other runway can remain in use since, in theory, you won't cross it. Keep in mind that this is dependent on the aircraft and airport in question considering the landing distances that are required.
For the record, I was flying a Piper Warrior and landing at Northeast Philadelphia Airport (KPNE) on runway 33 (length: 5000 feet) and the intersection with 6/24 is far off at the other end making the maneuver quite easy.

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Can I display the route which I have created with skyvector.com on google maps?
I really appreciate skyvector.com as a tool for initial route planning and providing me with a good lookout to the upcoming VFR flight, but honestly I'm not very satisfied with the FAA Sectional charts.
Using these charts navigation in some more congested areas is tough work especially as I'm not very familiar with the area yet. Its often useful to have a satellite picture of these areas to identify some helpfull landmarks, which might not be shown on the sectional.
Does anyone know how to transfer the skyvector route to Google Maps, or know an alternative to achieve a similar result?

My formatting cleanup may have made this a little less clear: Falk is using Skyvector as a first-pass (to be followed by actual planning with current chart data). Don't beat him up, he's actually Doing It Right :-)

No worries, but thank you for your help and your answer. Unfortunately I forgot to mention another poin. I don't have unlimited access to a computer and most of the time I'm working on an Android tablet.

Sectional charts do have identifiable landmarks identified - roads, water, towers (water towers easier, radio & cell towers harder to see unless you can find their lights), airports.  All the big stuff that's recognizable from a decent altitude. As you fly more, you will become more practiced at picking out the landmarks, and more proficient following the heading for the route you mapped out.  VOR helps a lot there, GPS helps a lot.

The other thing you will find is that google maps gives a view looking straight down, and while you are flying you will be looking ahead or off the side at an angle all the time. By the time you see something straight down it is too late, you're already passing it.

It's not web based, but you can use Google Earth for this — There are overlays for aeronautical charts available, and you can do all your flight planning by entering routes in Google Earth. Then just turn off the sectional and you'll have the Google Earth satellite imagery to work with.
As an added bonus (relating to your comment about mainly using an Android tabled): Google Earth is available as an Android application (and the plugin version of the chart overlay should work in the tablet app — I know it works in the iOS version of Earth)

BIG SCARY IMPORTANT DISCLAIMER
As the folks who publish that collection of charts will tell you, the charts are not always current and thus should not be used for navigational purposes (much like SkyVector).  You can of course replace the chart graphics with current ones (or locate a chart collection that is current) if you want to, but as you're only using this as a pre-flight planning aid and will be doing your real flight planning with current charts and data the ones provided in the Google Earth Library are probably "good enough" for your purposes.

Thank you for your 'bonus' I will try this out this evening :)

@Falk Google Earth has 3D coverage in some areas too - I believe in 7 and up you get (are forced to have) terrain, and lots of major cities have 3D buildings. It's an awesome toy to play with when planning a flight!

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Are there any aircraft with a nose wheel braking system?
I'm pretty sure that there are no aircraft equipped with a brake on its nose wheel, however two of my colleagues think there might have been.  Are there?
Aircraft with retractable gear of course have devices to stop the wheels from spinning when retracted,  but I'm asking about brakes used to stop or slow down the aircraft. Please don't consider aircraft with a tail wheel, gliders, experimental aircraft, or aircraft used for flight testing (certified aircraft only).

There are quite a few 727 that actually had nose wheel braking, however they utilized it only for maximum braking power. (Also they were proven to be ineffective and unreliable, so a service bulletin was written to remove the system).
Saab Gripens use nose wheel brakes to stop as well, the main gear sits farther aft than most planes, so there is quite a bit of weight on the nose gear, which makes it effective for stopping.

The Messerschmitt Me-262 had nose wheel braking as well. Back then, with the high landing speed of jets and no thrust reversers, the engineers wanted to give pilots as much stopping power as possible.
Here is a picture which shows the brake line running down to the wheel:

Abandoned Me-262 after the war (picture source)

Really interesting picture. My guess would be a forward deployment and that airplane was being cannibalized for parts.

@Terry: My guess is it has been taken by US forces when they discovered an abandoned makeshift airfield next to an Autobahn. Straight stretches of the Autobahn next to a forest (for hiding the planes and buildings) were used by several Me-262 units in 1945. Note the shattered windshield: I guess this particular plane had been damaged on the ground and was then cannibalized, as you said.

That is a really odd picture. There are jack-stands under the forward fuselage and left wing. What looks like an engine or bomb hoist holding up the right wing. It appears that there is some sort of wooden structure holding up the tail. The main gear look to be partially retracted, while the right gear wheel seems to be missing (hard to tell). These all strongly lead to the cannibalization theory, yet there are piles of ammo and a bomb under it, as well, indicating a hasty de-ordancing (is that a word?) prior to field repairs.

@FreeMan: Given that the engines are also missing, I would strongly guess cannibalisation.

Good question. With regards to mainstream commercial jets it might not do much and quite possibly be counter-productive.
Most (all?) Boeing and Airbus have about 95% of weight on the main landing gear (MLG), and thus the nose brake wouldn't be very heavily weighted, so not very effective. And for reasons of rotation, you don't want to load the nose very much, but rather have it pivot around the wings: where your lift is coming from.
You can then design the nose landing gear (NLG) to be quite light because it doesn't see a lot of load, nor take large braking reaction forces. And doesn't have any brakes/hydraulics etc. And the same with the fuselage strength: it's okay if the MLG take all the load: they are already in the "beefy" section of the plane, and there is no moment arm (torque) due to a big displacement from the centre-of-gravity (CG).
However if you started to have large forces on the NLG, you have it a long way from the CG, and thus a big moment: you'd need to take that into account when you designed the fuselage, making it stronger, and thus heavier. At the moment with no lift on the front fuselage section, nor any forces from braking, you can make it pretty light.
Having the MLG at the CG and also centre-of-lift from the wings (drag from spoilers) makes for a very stable system. Inherently stable too: the faster you go, the more drag from the wings, and thus more weight on the MLG, which they need.
I think this will be the case for all planes with propulsion from the wings: regardless if props or jets. If you have a prop on the nose it's a bit different because you need to beef that up anyways, so you don't mind the braking loads from a nose-wheel. But I don't see a lot of advantages for it. You add all the extra weight of the hydraulics and brakes, plus the weight from making a much stranger assembly. More complexity, more to service and go wrong.

Nose gear does have hydraulics for steering (except on small aircraft with free-castoring nose gear where differential braking is the only way to steer).

Braking force on nose gear won't have any larger moment than braking force on main wheels, because braking force is horizontal, so only the vertical distance from CG matters and that is the same.

Very interesting and useful information, however it doesn't actually answer the question of "are there any planes with NLG brakes?"

Once the brakes on the main wheels are engaged, the load on the NLG goes up, making a brake there rather effective. Only if the breaking is done by a chute or spoilers will the NLG see litte load through the deceleration.

Ahaha!! There IS in fact a non-ultralight aircraft out there that has its ONLY brake on the retractable nosewheel. That aircraft is the Verhees Delta. Uniquely, that aicraft has only two wheels (tandem, or bicycle undercarriage) and they are in line, one wheel at the front, retractable, and another, same size, 9 feet further aft, at the rear of the aircraft. There is a small bumper wheel at the tip of each wing.
The aircraft lands on the rear wheel - (the nose is very high, during landing as this is a delta wing aircraft). When it has slowed down enough the nosewheel leg makes contact with the runway and it can begin braking.

Welcome to Aviation.SE! Interesting airplane -- can you provide a link that shows us what this machine looks like?

Good catch! but it looks like it is an experimental (home built) airplane with a single one built, so it doesn't actually answer the question.

Well, I think the U-2 falls into this grou, and likely the B-47 and B-52 as well.

50+ B727-200 were equipped with nose brake systems. I have a B727 nose wheel cap with anti-skid. I have no idea what it is worth but it's a great conversation piece.


text:
How does a Mach Meter determine the speed of sound at a given altitude?
By my understanding, the Mach Number at a given altitude is calculated by dividing IAS by the speed of sound at that altitude. So how is this speed of sound calculated to display the Mach Number on the Mach Meter? Does the Mach Meter share the same pitot tube used to calculate airspeed?

Most modern jets use an Air Data Computer (ADC) to calculate (among other things) Mach Number.

Air Data Computer
An ADC is simply a computer which accepts measurements of atmospheric data to calculate various flight related data.
A typical ADC may be connected to$^1$:
Inputs

Static System Pressure
Pitot Pressure
Total Air Temperature (TAT)

Outputs (Calculated)

Pressure Altitude
Baro-Corrected Altitude
Vertical Speed
Mach Number
Total Air Temperature
Calibrated Airspeed
True Airspeed
Digitized Pressure Altitude (Gillham)
Altitude Hold
Airspeed Hold
Mach Hold
Flight Control Gain Scheduling.

Each of the inputs and outputs may be analog or digital depending on the design of the system, and are used for many purposes throughout the airplane.  Each output is a purely calculated value based on the various input measurements and data stored within the unit.
To answer your question about the pitot source for the Mach Meter: Yes, they use the same pitot and static sources as the airspeed indicator.
In the case of mechanical instruments, they are both connected directly to the pitot static system.
In the case of an ADC, the pitot static system is connected directly to the ADC and then electrical signals communicate the airspeed and mach number to the electric airspeed indicator and mach meter (or EFIS), which no longer require actual pitot static connections.

The Math
A simplified example for the Mach Number calculation$^2$ would be based on the pressure inputs:
$$Mach~number=5((PT/PS+1)^{0.2857}–1)^\frac12$$
Where:
$PT$ = Total Pressure
$PS$ = Static Pressure
The actual calculation makes corrections to the pressure data to compensate for installation errors and nonlinear sensor readings.
Note that it doesn't actually calculate the (local) speed of sound (LSS) in order to determine the current mach number, but with the TAT input and the calculated mach number, it could calculate it by calculating the outside air temperature (OAT/SAT) first:
$$SAT=\frac{TAT}{1+0.2\times{Mach}^2}$$
$$LSS=38.945\sqrt{SAT}$$
For example, let's say that the TAT is -36C (237.16K) and we are flying Mach 0.80:
$$SAT=\frac{237.16}{1+0.2\times0.8^2}=\frac{237.16}{1.128}=210.25°K=-63°C$$
$$LSS=38.945\sqrt{210.25}=38.945\times14.5=564.70knots$$
Again, these are simplified formulas because the actual ones would consider sensor error, etc.

$^1$ List of inputs and outputs obtained from Air Data Computers.
$^2$ Formula from TAT Sensor Operation and Equations.

Thank you for the good answer! Is it possible for the Mach meter to determine speed of sound from the TAT alone, without any input from the static system?

@shortstheory Unfortunately, no but not for the most obvious reason.  You need to know SAT in order to calculate the speed of sound, but in order to calculate SAT you need to know your airspeed in order to calculate the ram rise (difference between TAT and SAT) caused by flying at high speed.  For all practical purposes, the ADC needs all three inputs in order to make any meaningful calculations....

@shortstheory I added an example calculation that could be used to determine the speed of sound.

Thanks again! But, I have another question. Mach 0.8 at such LSS means an IAS of nearly 450 knots. That's much higher than the 250 KIAS/Mach 0.86 of my 747 in FS2004 at 40000 feet ;) Where does the difference arise from?

@shortstheory Ahhh, it's only a slight difference in terminology, but Mach 0.8 at this LSS is a TAS (True Airspeed) of nearly 450 knots, not an IAS (Indicated Airspeed).  Because of the lower air density at high altitudes, the airspeed indicator reads lower than it would at the same speed down at a lower altitude.  An ADC will normally calculate TAS as well (see the list of outputs above).  You probably have a TAS readout somewhere in the cockpit of the 747 that you can use to verify this.  :)

Eh, I forgot where it is on the 747 panel, and I'm using ubuntu atm. But thanks for everything, +1!

An (analog) machmeter looks something like this:

So it's more like an more complex version of the airspeed indicator, in this case correcting for the altitude in the process. That being said, I found this extract apparently from an FAA publication:

Some older mechanical Machmeters not driven from an air data computer use an altitude aneroid inside the instrument that converts pitot-static pressure into Mach number. These systems assume that the temperature at any altitude is standard; therefore, the indicated Mach number is inaccurate whenever the temperature deviates from standard. These systems are called indicated Machmeters. Modern electronic Machmeters use information from an air data computer system to correct for temperature errors. These systems display true Mach number.

Most systems today use more detailed data from sensors to give a correct value through a variety of (complex) calculations.
A little more discussion is available on PPruNe.

Side note: Speed of sound ($a$) itself is solely determined by temperature (that being said, you are able to determine it from pressure, as pressure is a function of temperature) hence the problem with the analog system above.
For air:
$$a=\sqrt{R{\gamma}T}~m/s$$
Where:
$R=287$  Specific Gas constant [dimensionless]
$\gamma=1.4$  Specific heat ratio [dimensionless]
$T=$ Absolute temperature [K]
Remember that you're reading off indicated airspeed [IAS] in knots in the cockpit, which is not the same as True airspeed [TAS] converted to m/s, in case you're trying to work out your mach speed manually ($M=\frac{TAS}{a}$)
For use without knowledge of airspeed & temperature, Wikipedia gives the following formula for subsonic flows:
$$M=\sqrt{5((\frac{P_T}{P}+1)^\frac27-1)}$$
Where:
$P_T=$ Total Pressure
$P=$ Static Pressure

The gas constant of dry air (R_d) is not dimensionless, it has units of J kg-1 K-1.  You are correct on the other units so that RgammaT have units of m2 s-2, and the correct units m/s when the square root is taken.

A Machmeter does not determine the speed of sound. It doesn't even need to:
$$Mach~Number=\frac{P_T-P_S}{P_S}$$
Mach number is simply the ratio between total pressure minus static pressure, divided by the static pressure.
Here is why:
$$Mach~Number=\frac{TAS}{LSS}$$
The Mach number is true airspeed versus the local speed of sound
$$TAS=IAS\sqrt{T}\div\sqrt{P}\div16.97$$
Converting indicated speed to true speed, we need to multiply with the square root of absolute temperature (in °K)
$$LSS=38.94\sqrt{T}$$
Also the local speed of sound is directly proportional with the square root of absolute temperature (in °K)
If you divide $TAS=IAS\sqrt{T}\div\sqrt{P}\div16.97$ by $LSS=38.94\sqrt{T}$, the $\sqrt{T}$ will cancel eachother out.
$$Mach~Number=\frac{IAS}{\sqrt{P}x}$$
IAS we already have, it's dynamic pressure minus static pressure, and $P$ is just static pressure, or, like I said in the beginning:
$$Mach~Number=\frac{P_T-P_S}{P_S}$$
See, no thermometer... only dynamic and static pressure.

This answer does not stack up. For instance @ ISO atmosphere sea level, speed = 200 knots = 102.9 m/s, a = 340.3 m/s. M = V/a = 0.3, but your equation results in M = 0.06

which equation are you using to get 0.06? remember that T is in Kelvin and P must be in Pa. Did you notice there is a constant 'x' multiplier there in the equation?

$p_t = p_s + \frac {1}{2} \cdot \gamma \cdot p_s \cdot M^2$. All units are SI units, pressure in Pa, V in m/s. I don't see a constant x in your first and last equations.

ok, but how do you use that to get to pt from V, w/out using M? .. you cannot use M to find pt that you then use to find M, .. unless I'm missing something

Why not? The equation must still hold. Both sides of the = sign must be equal. Start with a V, speed of sound is known, M follows, $p_t$ follows, compute right side of equation which must be equal to left side of equation.

A derivation of the correct equation is given here


text:
Why do some aircraft require type ratings to fly them?
I know it might seem like a silly basic question, clearly some of these aircraft are awfully complex.  I doubt anyone questions the idea of a type rating for a 747-400.
But where is the line between, "sure, if you can fly you can probably fly this one" and, "you need to know how to fly this plane in particular".  What sorts of functionality cause that sort of distinction?

The FAA specifies the airplanes that require a type rating in 14 CFR 61.31:

Sec. 61.31 - Type rating requirements, additional training, and
  authorization requirements.
(a) Type ratings required. A person who acts as a pilot in command of
  any of the following aircraft must hold a type rating for that aircraft:
(1) Large aircraft (except lighter-than-air).
  (2) Turbojet-powered airplanes.
  (3) Other aircraft specified by the Administrator through aircraft type certificate procedures.

So in short, large aircraft (meaning that they weigh more than 12,500 lbs.), aircraft having turbojet engines, or any airplane deemed suitable complex that it needs it by the FAA need a type rating in order to fly it as PIC.

Mkay.  What causes a plane to be "suitably complex"?  I mean, the other two are pretty straight forward, but that one...not so much.

Well, that is solely determined by the FAA during certification.  Basically if they feel that it is more complicated than a "normal" airplane, then they will require a type rating.  These are generally for exceptional cases though, and the first two cover most type ratings.

Is there a more concrete description than 'Large aircraft'? Dimensions, or something? What makes a plane large? Who decides?

@flyingfisch I put that in my answer.  The FAA definition of Large is not determined by size, but by weight.  Greater than 12,500 lbs.

@flyingfisch 14 CFR 1.1 Large aircraft means aircraft of more than 12,500 pounds, maximum certificated takeoff weight.

It might be fun, if someone wants to take the time, to provide a couple of examples of those "exceptional cases", that's really what I'm curious about.

Actually, I looked through their list of type ratings and didn't see any that didn't fall into one of the first two categories (although I could have missed some :) ).

Aw, well. It was an interesting questions that had a simple answer, that happens sometimes ;)

Just so you know, for one of their recent models (747-8, I think) Boeing was able to shorten the training period because the types were so similar to previous models. Different enough to need training, similar enough to cut training in half.

They appear to be incredibly consistent with the designation. For example the P51 Mustang (max take off weight 12,100lb) does not appear to require a type certification, while the P38 Lightning (max take off weight 21,600 does). Not that I would want to fly a P51 without a type check.


text:
Accelerated Stall, what is it, how do you prevent it?
I've read a lot of NTSB crash reports regarding small, GA aircraft (just trying to figure out what went wrong and what to avoid.)  There seem to be a lot of reports that talk about "low altitude high speed stalls" (accelerated stall) happening on approach.
What is an accelerated stall, and how is it created?  What is the best way to avoid one?  As they seem to cause a lot of GA accidents...

Probably the most devastating example in the recent history is Air France 447. All it took at cruising speed is an unwarranted nose up input for a few seconds. Many factors went into that crash, but at the end of the effort the aircraft stalled out of the sky from 38,000 feet. After the stall, angle of attack was never corrected and maintained at 35 degrees until impact; all-the-while first officer applied TOGA power thinking that alone was gonna get them out of trouble. Very sad.

@user1599043 Actually, he held the nose up input for a lot longer than a few seconds and it gradually bled off airspeed.  It wasn't really an accelerated stall, but more of a typical stall.  Still sad....

Short Version
An accelerated stall is a stall that occurs at an airspeed higher than normal due to a higher load factor (g loading).
Longer Version
When an aircraft is in a bank or when pulling back on the yoke quickly, the wing has to create additional lift to support the aircraft since the load factor has increased.  This increases the angle of attack beyond the critical angle of attack (the point where the wings can no longer produce enough lift to support the airplane) and the airplane will stall even though it is above the normal stall speed for the airplane.

Example
Let's say that we are flying an airplane that has a "normal" stall speed of 60 KIAS.  If we start a 60 degree banked turn, this will increase the load factor on the airplane by two (2g) and will increase our stall speed by 41% to 85 KIAS.

A fairly common cause of this type of accident is when a pilot overshoots the runway centerline during the turn from base to final and steepens the bank angle in order to "turn faster" to get back on final.  Because the stall speed goes up, they can stall the airplane even though they normally never even come close at their current airspeed.
Instead, they should have just maintained the current bank angle that they had (instead of steepening it) and continued the turn back to re-intercept final using normal maneuvers.  If that couldn't be done in the amount of altitude/distance available, then simply go around and come back to try again (while making the proper wind correction in the turns this time).

Excellent, that makes perfect sense.  That does sound like a lot of the cases I read.  What was always odd about those cases is that it always seemed to be a pilot with ATP and a lot of hours...  I wonder if flying large aircraft creates some different habits?  Or perhaps a jet is less susceptible to high speed stalls on final?

Well, maybe it is because there is a second person there in a jet to keep an eye on the flying pilot, or that they are normally flying an instrument procedure in the jet which usually keeps them out of that situation altogether....

Logical, I suppose.  Granted, this is just my biased observation...

@JayCarr: Also I believe large planes generally fly with higher margin to stall. The approach speed is 1.3×Vₛ (to be flown until threshold) and the heavy plane will lose a few knots during flare while small plane pilots are usually taught the stall warning should go off during flare, so they may leave themselves less margin. Plus there is nobody shouting "go around" at the private pilot when the speed decreases below the Vₐₚₚ. Pilots with more hours are often more prone to this because they are more confident, trust their estimates more and don't watch their speed as carefully.

@JanHudec- So...the moral of the story is to fly with a higher margin to stall further into the approach?

@JayCarr: The trouble is that on unpaved or short runway you need to balance it with the need to land as slow as possible. So it's more like watch the speed carefully and be ready to go around if you find yourself under the profile.

Additional risk stems from the fact that in a turn you are more likely to be slightly uncoordinated and end up in a spin.

@JanHudec True, most jets have yaw dampers too and they keep you coordinated without pilot input.

Another factor may be that jets keep power on almost all of the way to touch down and then bleed airspeed off relatively slowly.  This is different than in a light airplane.

@Lnafziger: Propeller-driven aircraft also keep power almost all the way to touch down exactly because they bleed airspeed of faster when power is retarded. They also respond almost immediately to increasing power setting (piston engine has weaker relation between rpm and power and turboprops always have constant-speed propellers to keep them at high N1 during approach), so  they are easier to handle than jets.

@JanHudec They also respond almost immediately to power reductions (whereas jets don't) so my point was that when they reduce power in the prop airplane they will slow down faster than they are used to.  And not all prop pilots keep power in on final (some prefer to always be within gliding distance of the runway).  :-)

@PeterKämpf Now that he has changed his question, I can do that.  For the record, I didn't call it a high speed stall, that was the term that the OP used, and he probably used it as short-hand for a stall at a higher than normal airspeed (an accelerated stall). As far as giving it a different name, as I mentioned in my previous comment, I don't believe that "high speed stall" even has an actual definition, does it?  I think that you are doing the same in your answer, but answering the wrong question...

B-52 accelerated stall (warning: video shows a deadly airplane crash):
https://youtu.be/182AepOJjMs


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Does the MD-80 seat layout affect the balance of the aircraft?
The MD-80 family has a 2-3 seat layout as seen in this image:

Does the layout have any influence on the balance of the aircraft and if so what is used to balance out the weight?
Image from seatguru.com

Based on this diagram—found in this Boeing MD-80 series document—we can estimate the average lateral center of gravity along the buttock line for each passenger seat:

Using the seating diagram that you provided in your question, I estimated the average center of weight along the buttock line for A, C, D, E, and F to be -50, -30, +10, +31, and +51 respectively.
We can use the following formula to calculate the lateral CG for the full load of passengers.
Where:

$A_w$ is the sum of all weights for passengers in A seats (and so also for C, etc)
$A_a$ is the lateral moment arm location along the buttock line for passengers in A seats (and so also for C, etc)
$C_G$ is the solution to the lateral center of gravity for all seat loads.

$$
C_G=\frac{(A_w\cdot A_a)+(C_w\cdot C_a)+(D_w\cdot D_a)+(E_w\cdot E_a)+(F_w\cdot F_a)}{A_w+C_w+D_w+E_w+F_w}
$$
If we assume that every seat depicted in the question diagram is occupied by a 170 lb person, the solution $C_G$ is $3.4$—that is, 3.4 inches right of the center of the cabin, under the left side of seat D.
3.4 inches is very little variation in lateral CG, especially for so large an aircraft.
Other load considerations notwithstanding, I don't think the assymetrical seating arrangement is cause for concern.

Your first link not worketh.

Isn't it simpler to ignore the two outer files on each side since they cancel out?   You're left with the middle one being half the aisle width off centre.  3 tons of people sounds like a lot but the lever arm is only 30cm so the torque isn't so much.  I suppose if it started rolling too much they could announce that people in the odd numbered seats must stand up in the aisle, but I've been on them a few times and it's not happened yet.

@BlokeDownThePub: They don't cancel out for the purpose of computing the centre of gravity. If you removed all the window seats, you would end up with a CG that is further from the midline.

@BlokeDownThePub As TonyK said, they will not cancel out. First, if we were to assume that they had the same absolute value buttock line position (which I do not believe they do) the sums would not work. To illustrate this point, lets remove both outer seat pairs from the CG calculus. In this case, the CG would be centered on remaining the D aisle seat. Secondly, the seats do not seem to fall symmetrically about the aircraft. Consider that the diagram depicts the aisle at 19 inches and the seats at 22 inches of width.

As you wrote, symmetrical rows play a role in minimizing the unbalance, this is the same for the mass in the hold as well as the structure, assuming they are laterally balanced. Taking them into account should lead to an even smaller moment.

The "D" seat is very near the aircraft centerline, and thus the roll axis. Since roll moment is the distance from the roll axis times weight, if the distance is small, the roll moment is very small. What roll moment is created can easily be canceled by a little bit of aileron trim if needed.
The water tank on MD-80s is also placed on the LH side of the cargo. Full this is about 350 lbs, further from the centerline than the "D" seats. Also the LH side has an extra service door in the aft and the front LH door is larger than the R1 door, although the RH side also has the cargo doors.


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Can a private pilot fly commercial-type aircraft?
Lets say that a private pilot owns a large commercial-type aircraft like the B777. Can he fly such a plane for himself as a private plane?

The fact that you own the aircraft is not significant for your ability to fly it. You may as well rent it or borrow it.

Your certificate level determines the type of operations you can conduct*, not the type of airplanes you can fly.  The types of airplanes you can fly are determined by your category, class and type ratings.
If you have a Private Pilot certificate with an instrument rating (not sure if this is required, but you'll want it either way) and an airplane multi-engine land category and class and a B777 type rating, then yes, that person can operate the aircraft privately.
He will also need an SIC and he will only be able to collect the pro-rata share of operating expenses from people he carries.  He will also want to be careful in who he lets on his plane to avoid operations that are regulated under 14 CFR 119.
An example of such a pilot is John Travolta, who has a private pilot certificate with the following ratings:
PRIVATE PILOT
  AIRPLANE SINGLE ENGINE LAND
  AIRPLANE MULTIENGINE LAND
  INSTRUMENT AIRPLANE

Type Ratings:
P/B-707 P/B-720 P/CE-500    P/CL-600    P/EA-500S
P/G-1159    P/HS-125    P/LR-JET

Limits:
ENGLISH PROFICIENT.
AUTHORIZED EXPERIMENTAL AIRCRAFT: SO-G2.
B-707 SIC PRIVILEGES ONLY.
CE-500 (VFR ONLY).

* Private pilots may only collect pro-rata operating expenses, Commercial pilots may be paid to act as PIC and ATPs may act as PIC in 14 CFR 121 operations.

What you posted shows a SIC only limitation. Is travolta not certified to be PIC in his 707?

@GregTaylor for the 707 type, yes, but his others are not similarly restricted.  The VFR only CE-500 is odd, but perhaps he's just using that for cross-town commutes.


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What was Boeing's competitor to the C-5?
Doing some of my airplane learning, I have come across a point many times that the engines for the Boeing 747 (at least partially) came from Boeing's competitor to the C-5 Galaxy. What was this plane that Boeing had made to compete with the C-5?

I'm not getting this question, the C-5 is military and the 747 is civilian. Closest thing is the C-17 GlobeMaster III I think.

@RonBeyer I am asking about Boeing's competitor for the C-5 contract before the C-5 was. It is just like the YF-23 was the competitor to the now F-22.

According to the book The C-5A Scandal the 747 came out of Boeing's failed bid for the C-5 contract.

In 1964 the US Air Force awarded study contracts for a new cargo aircraft to Boeing, Douglas and Lockheed. The aircraft was called CX-HLS. ( cargo experimental - heavy logistics system)
Lockheed won with the C-5A design. Boeing's design was never built but would have looked like this:

source: Widebody Aircraft Chronology

Source: Boeing archives

Source:TDPRI.COM

Based on the TDPRI link, it appears that Boeing called theirs the "C-X", which seems pretty darn generic...

Boeing called the aircraft CX-HLS( Cargo Experimental - Heavy Logistics System). It never entered production though.
The below image is of a scaled down model of the CX-HLS.

Source: Boeing Archives - Bellevue, Washington USA
Details about the CX-HLS can be found here


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What is this runway marking on Kirkwall Airport's runway?
What are these markings on Kirkwall Airport's runway? From where they are on the runway, they seem like the touchdown markers, is that what they are?

Aiming point. This is where aircraft should touch the runway. Its location depend on landing distance available (LDA). According to Canadian regulation, on common runway length (4000 to 8000 FT) this is located at approximately 1000 FT down from threshold. On longer runways it is located at approximately 1300 FT. As mentioned by “ymb1” this type of marking is used in UK. In North America the aiming point looks like a rectangular shape. For you information, here’s a drawing of Canadian regulation.

During a precision approach (ILS or RNAV (GNSS)), the vertical guidance (usually 3 degree slope) should bring the aircraft close to this point. Even more, the precision approach path indicator (PAPI) if followed as recommended (2 RED / 2 WHITE) should also bring the aircraft close to the aiming point.

The picture is of an airport in the UK, not Canada.

This isn't an answer to the question.  I don't see the markings in the OP in your diagram.

Aiming point is not where aircraft should touch the runway. It is the point where your eyes would land if you continued on the approach. The wheels, being lower and farther back, would touch significantly earlier on the runway. This, of course, ignores the flare, like all the aiming/glideslope information, but the relation between the aiming point and the flare distance is not defined.


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What are the aerodynamic differences between the different types of slotted flaps?
How do the different types of slotted flaps (Single, double, and triple as well as a non-slotted one just for comparison) differ aerodynamically? I read in Joe Sutter's book 747 that a double-slotted flap design for the 747 had 8 knots higher speed than a triple-slotted one. How does that work?

which speed does the book refer to? stall speed? cruise speed? second-segment climb speed?

@mins Low Speeds

There is a comparison of flap type efficiencies in this answer and a smaller set, but with full lift graphs in this one.

@JanHudec They compare the different types of flaps, I just want the different variations of the slotted flap.

Related: (1953) study.

The high energy air that passes through the slots prevents boundary layer separation over the top surface of the flap. Though, this increases drag but it improves the stall characteristics of the airfoil. In other words, we can have higher maximum lift coefficient (Cl_max) by having higher maximum angle of attack (alpha_max). So, higher the number of slots higher Cl_max one could reach, there should be a limit. In case of a fixed weight of an aircraft,a higher lift coefficient implies that one can fly at lower speed before it stalls. A comparison between different types of flaps (up to double slotted)can be found in the book Aircraft Design and Performance by Anderson.
Source:
The same book.

just one detail: More slots will not necessarily give a higher maximum AoA, but a higher distance between the zero-lift AoA and the maximum AoA. More slots translate into more camber and a more negative zero-lift AoA. Also, each slot allows to stretch out the flaps more, so the Fowler effect of an increased surface area is increased.

There is no high energy air in slots,only if you have external source  like air pump...total pressure cant be higher then in freestream

@JurgenM Total energy cannot be higher in the free stream, but it can be lower - in the boundary layer. Slots help in restoring this BL energy. While the energy of the air in the slots is not higher than the total energy, it is relatively higher when compared to that of the BL, and that's what he meant with "high energy air".


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Can a passenger request to visit the cockpit during a flight?
As a kid, I remember going to the cockpit of a plane with my mom (I believe the airline was AOM French Airlines).
Is this still possible now?
I understand the answer might differ with different airlines/countries, I'm mostly interested in European and Asian airlines/countries, specifically Japanese.
Is there some specific regulations for international flights, or does it depends on departure/arrival locations?

Related question

You can ask...

For EU and USA, this question has already been answered here. This is also a related-ish question.

Pity they don't put a camera in the cockpit that I could watch as a channel on the in-flight entertainment system... with the audio channel hooked up to the radio.

@romkyns What, and have the passengers panicking because they've misinterpreted something the pilots said, or some minor problem with the plane as meaning OHMYGODWE'REGONNACRASH!!!

This stopped after the World Trade Center attacks.
You might be able to get a kid in briefly before the plane takes off.

Or after it lands.

I know I did this as a kid before takeoff, and I'm pretty sure it was after 9/11.

@cpast Which company/origin/destination ?

@Antzi I think it was Delta; it was years ago, though, and I think I was still in elementary school at the time (i.e. like 10 years ago), and I don't remember origin/destination, other than "probably US domestic flight, because we don't go internationally that often". What I remember is that I got to see the cockpit, and I think the pilot let me request more printer paper, and I got a little set of "junior pilot" wings. That's about what I remember.

My eight year old daughter was in the cockpit of an A320 and an A330 on the same day a little less than a year ago. The A330 captain even gave her the flight map! Both were after landing, once in Spain and the second in Israel.

My guess is that it's easier to do this the younger the kid is.

This is inaccurate. As an adult, I visited the cockpit during a transatlantic flight in 2008. However, I did have to return to my seat before the plane arrived over Canada because of post-9/11 regulations.

I attended a talk by an A380 captain who works for BA. I asked him this question. His answer was a very enthusiastic "YES", but unfortunately only on the ground. He also said that he'd allow an adult just as well; doesn't have to be a kid. He was saddened that he could no longer let enthusiasts and especially aspiring children to experience the cockpit in flight.

@romkyns The problem is that before the flight, as the passengers file in, there are only a few minutes before take off and the crew is busy getting ready for departure. Thus, the window for a cockpit tour is pretty brief.

In the early/mid 90s, my parents and I had a flight where the first leg only had 3 passengers: us!  I clearly remember being in the cockpit with the pilots who had me help start the engines: when a certain gauge reached 20 I threw a lever all the way forward.
I'm pretty sure that post-9/11 this just wouldn't happen.

@cpast "I know I did this as a kid before takeoff, and I'm pretty sure it was after 9/11." This makes me feel old.

When I was getting my PPL (Pre 9/11) Air Canada let me ride in the jump seat for a half a flight from BOS-> YYZ that was amazing. More recently I got to sit in the pilot seat of an EasyJet flight from TLV->AMS while we were waiting for some weather to clear. I stuck my head in as I was boarding and said hi to the co-pilot. He invited me to do so.

Generally speaking, you will not get very lucky unless you are a pilot yourself and are carrying your license with you (and even then, this is airline dependent). If this is the case, you might even be able to ride in the jump seat for a portion of the flight
On the ground a lot of pilots/cabin crew are friendly and would be happy to show you around. Use common sense though and ask to go after landing for the best chance - before take off the crew will be quite busy with pre-flight checks.

By 'pilot' here do you mean 'airline pilot?' Most pilots, of course, are not airline pilots. Also, is there a specific area where you happen to know this to be the case? I have never heard of this being a possibility for private pilots here in the U.S., but if it is, I'd really like to know.

I'm based in South Africa and having a cursory glance at various local aviation forums, it appears that this happens occasionally here. From the language used, it appears learner pilots have the quite a bit of luck, with the airline pilots being happy to show the up and coming pilots what its like up front on a big jet and answer any questions they may have.

Awesome! Thanks. You might want to edit your answer to include that information (specifically the parts about it being in South Africa and student pilots having a better chance.) Also, if you could include link some of the forum citations, that would be great. As a student pilot, this makes me want to visit South Africa. - lol

Usually the cockpit is restricted during any phase of flight besides boarding/de boarding. This is usually the case with major airlines with any destination in Europe, or the U.S., not sure abut Asia though

As for Asia: No, same rule as US/Europe - except you know the right people, then it's possible... One of my wife's friends is a pilot of a major airline. We've been to his cockpit several times.

Can I ask to go to the cockpit during a flight ?

Of course you can. The answer will be a firm "No" and you will be watched very closely by the cabin crew and any plainclothes security onboard until you land. It's possible the police will be waiting for you.

Can I ask to go to the cockpit after a flight ?

Of course you can. If it's a longer stop the crew are often happy to show off their office. If it's the end of a 12 hour intercontinental flight with a bad-weather landing they might decline.

I'm curious - can you provide any instances of the police talking to someone after landing due to a request to visit the cockpit?

@colti It wasn't in the media but I know a couple of years ago two student pilots had that treatment after asking to visit in flight. I do wonder if the police would have been called if they were white students...

About 30 years ago I did this during flight when I was a kid. These days I manage to do it for my daughter but only while boarding.

It's a security and/or safety risk to allow any persons in the cockpit these days.
This accident happened as a pilot allowed his children to take control during flight.

With the autopilot active, Kudrinsky, against regulations, let the children sit at the controls.

I've seen flight crashes, in which persons in the cockpit have interfered with the flight in one way or the other.
Based on these experiences, unauthorized persons in the cockpit is totally not allowed these days.

I think you can reference this accident as well, if you want. "[The accident] was in part caused by the airline electing to not follow the Sterile Cockpit Rule and that a passenger was sitting in a cockpit jump seat during the flight."

9/11 is much more significant here than Aeroflot 593.

You probably need to qualify that last sentence; those particular experiences did not drive changes in cockpit access; and it's clear from other answers here that it is not the case that unauthorized persons are "totally" not allowed in every cockpit.


text:
Why doesn't an airfoil rotate?
As far as I understand, two of the reasons how an airfoil generates lift are Coandă effect and downwash. However, if there is upwash near the leading edge, why isn't an airfoil rotated by the two forces (upwash at the front and downwash at the back of an airfoil)?

Airfoil lift and Coandă effect are similar but strictly distinct phenomena. Coandă effect describes situation when a distinct stream flows flows around an object surrounded by static, possibly different, fluid. On the other hand lift occurs with homogeneous oncoming flow.

It does rotate:http://en.m.wikipedia.org/wiki/Pitching_moment

Ditto tpg2114.  For example, on most modern airliners the wing in flight causes a negative pitch (nose down) rotation.  In level flight the horizontal stabilizer produces a down force (positive pitch with longer moment arm) to balance these rotations.

@mrbcuda That pitch-down moment is caused by the center of gravity being forward of the center of lift (i.e. wings are aft of the CoG,) not by forces acting on the wings themselves in different directions. If the wings were positioned such that the CoG coincided with the CoL, then that moment wouldn't exist. Similarly, if the wings were positioned such that the CoG were forward of the CoL, there would be a pitch-up moment.

There are no two (aerodynamic) forces. Force is not caused by the up- or downwash itself, but by the change from up- to downwash. The air is only accelerated downward and this change in it's momentum causes an upward force on the wing everywhere, there is no aerodynamic down force. Due to the pressure distribution however the centre of lift is about quarter-chord for subsonic airfoils and changes somewhat with changes in attitude.
There however obviously is a down force, gravity. If the centre of gravity and centre of pressure are offset relative to each other, there is a moment that rotates the aircraft. That's why a plane needs horizontal stabilizer with elevators that allow shifting the centre of lift by adding a bit of controllable lift far from the centre of lift of the main wing.

It does rotate. In fact, in aerodynamics, this rotation force has a name: pitching moment. Pitching moment changes depending on the angle of attack.
Airfoils can be designed with almost any pitching moment and still generate lift, from positive (trailing edge rotates upwards as you describe), to zero (no rotation) to negative (trailing edge rotates downwards). In general, high lift airfoils have positive pitching moment (because, as you noticed, downwash caused by the Coanda effect increases lift). But zero and negative pitching moment airfoils are useful for tailless and flying wing aircraft. Helicopter blades are long, narrow and flexible so must use zero pitching moment airfoils to prevent them twisting.
If you use an airfoil with a positive pitching moment you need something to counter the rotation. For most aircraft the solution is the horizontal stabilizer (the tail). An alternative solution is the canard.
For chevron shaped (V-shaped) flying wings and tailless planes the solution is to twist the wing (in regular planes it's called washout but flying wing designers call it "twist"). The twist causes the wing tips to have less angle of attack (sometimes negative) so that the overall rotation force for the entire aircraft is zero.

Perhaps you're thinking of something like a fanwing, which incorporates both centrifugal fans and an airfoil into the wing

Pretty interesting concept, thanks for pointing it out. I've been thinking about designs with a compressor spanning the almost entire wingspan before (prior interest), but while they work, speed is an issue. Wonder if the fanwing people can find a market.

You are right, lift is concentrated in the forward part of an airfoil, especially at high angle of attack. If you take a single wing and drop it, it will start to rotate around its spanwise axis quickly. This effect is called autorotation (not to be confused with autorotation in helicopters, though!). Model aircraft pilots should have seen that before: When a model breaks apart in midair, its wing will in many cases autorotate to the ground.
In an airplane this can be avoided by shifting the center of gravity near the center of lift and by adjusting the lift distribution either with a tail surface, a canard or with wing flaps such that the center of lift is exactly over the center of gravity. Flying wings can do this without a separate tail surface, but need special airfoils which show little change in the location of the center of lift over angle of attack.
Below you see the pressure acting on an airfoil symbolized by arrows (yes, that is XFOIL at work), and arrows pointing at the airfoil indicate pressure higher than ambient air pressure, and those pointing away from it indicate suction. Their length is proportional to the magnitude of either pressure or suction.

This shows that the forces in the forward part of the airfoil are more intense than those in the rear part. The aerodynamic forces which depend on the angle of attack are focused at the quarter chord point, and without added balancing masses of a fuselage and engines, will be forward of the wing's center of gravity.
Of course, this is strictly true only as long as we have no flow separation. Once rotation starts, separation is unavoidable above the maximum angle of attack. However, inertia will keep the wing spinning until the trailing edge points forward and a similar pressure distribution develops. This gives the rotation another push and keeps the wing spinning over a full revolution, and the whole process starts over.

Because the horizontal stabilizers provide a counter force to the plane in its entirety so the pitch remains stable.

If the airfoil is cambered (in other words unsymmetric looking sideways) it does indeed rotate. This is why the control surfaces are needed with a preferably long moment arm and why building flying wings is difficult.

There is not really big difference between cambered airfoil and noncambered one flying with non-zero angle of incidence.


text:
In turboprops, is there a mechanism to prevent the blades running in opposite direction?
Because, it might lead to reverse flow of air and could stall the plane. Is there some mechanism to prevent this?
There was a crash in Ireland a few years ago, when the pilot of a small charter plane accidentally put one of the turboprop's in reverse.

Are you referring to reverse-thrust as normally used on landing to slow the aircraft?

yes i am referring to that

The blades still run (i.e rotate) in the same direction, the blade angle is changed to produce reverse thrust. On the Pilatus PC-6 turboprop you can use beta-pitch (zero-thrust) when you want a near-vertical descent.

Most manufacturers realise this is a serious risk so most are 'gated' in that you have to do something to get them into this range. This is like pulling up the reverse tabs on the throttle levers on the B737 or A320 that is only possible when they are in the idle position.

ATR 42/75: Has something called Idle Gate that prevents throttles from going back too far when engaged.
Dash 8: Has gated throttles that requires pulling up a pin to get them into reverse.

There is to the best of my knowledge nothing (in general at least) to stop to the thrust from being 'reversed', or more specifically to angle to blades to what is known as the 'beta' range, if you are intent on doing it. Unlike jets where doors open for instance and the process is a little more complex, this is just rotating the blade below a certain angle.

I found the accident report [PDF] for the Manx2 flight. There were multiple things going on here rather than a single cause:

Generally bad piloting for putting them in a late go-around situation to start with.
There were errors in the engine, causing a thrust mismatch.

This meant that one engine proceeded to continue into negative thrust.

It does then not come as a complete surprise that you manage to flip the plane over from considerable unsymmetric thrust when you are close to stall speed.

Negative TORQUE isn't exactly the same thing as negative (or reverse THRUST). Negative torque means that instead of the engine driving the propeller, the propeller is turning the engine. That's a lot of drag, but it isn't reverse THRUST. When you use reverse thrust, the engine is driving the prop, and you'll have considerable positive torque - but the thrust itself is negative (due to blade angle). I'm not sure the report being quoted is the BEST example of the harm of going into the Beta range in flight (problems happened when they came OUT of Beta), although I'm sure those could be found.

As was pointed out in a comment, the "reverse thrust" of the Beta range, which is used to slow the aircraft on landing and which certainly can do disastrous things if deployed inadvertently in-flight, is a matter of propeller angle and not the direction of rotation of the engine.  If it is running, the turbine engine is going to turn one way and that way only; all sorts of things wouldn't work right to try to run it turning it backwards.
The place where a prop and engine might try to go backwards is with a shut-down engine and a feathered prop.  If the engine isn't running, then the lubricating oil that normally keeps moving parts lubricated and cool, isn't being supplied, and it's important that there NOT be any movement.  With the prop in the "feather" position, it's generating a slight force to turn the engine the "wrong" way, and on the system I'm familiar with, we had a "prop brake" that would hold the prop stationary at that point.  If you took the blade angle out of feather, then the brake would allow the propeller to turn the "right" way -- as in a restart attempt.
So to directly answer the question as asked, yes you can find turboprop engines with a system to prevent them from rotating the wrong way.  But the reason for this has to do with preventing unwanted (unlubricated) rotation after an in-flight shutdown, rather than anything related to reverse thrust.


text:
Do any ejector seat designs include controllable thrusters?
Are there any ejector seat designs with built-in thrusters, or boosters, which the pilot can deploy to maneuver himself out of, or away from enemy territory?
A use case might be that after the parachute is deployed, the ejected pilot uses the thrusters to change direction or propel himself in a different path.

A rocket big enough to take them out of enemy territory would be too big to be practical.  The motors are only to allow safe parachute deployment.

Yes, modern ejection seats have rocket motors, but they are only for propelling the seat away from the aircraft and upwards, high enough for safe parachute deployment. An escape from enemy territory is not possible with them.
They are controlled by a guiding system inside the seat, but not by the pilot. Ejection by itself is stressful enough, controlling the seat would be too much for any pilot.
Nevertheless, this is impressive technology. Even when ejected pointing downward at low altitude, the seat will change course and lift the pilot up.
The seat featured in the linked video is the Russian Zvezda K-36, the only ejection seat which meets or exceeds all US Air Force performance demands.
To get downed pilots out from enemy territory the armed forces use helicopters. To build such a capacity into the seat would make it impossibly heavy and dangerous - would you like to sit on hundreds of kilos of jet or rocket fuel while being shot at? Just for the chance of flying home after ejection? And how deep are you allowed to fly into enemy territory for the system to work? No, this idea is a dead end.
The most promising concept today is to separate the pilot from the airplane, so he sits thousands of miles away while his craft flies deep into enemy territory.

"Would you like to sit on hundreds of kilos of jet or rocket fuel while being shot at?" No. In fact, I'd rather avoid even the being-shot-at part.

To get downed pilots out from enemy territory the armed forces use helicopters. More often and more effective is the use of PJs (with or without helos)

No, the small rockets present in an ejection seat are only for zero-zero ejection situations.
Although one can do some maneuverability using parachutes but the purpose of parachutes isn't traveling over land, but descending at a survivable speed.
The scenario you mentioned is ejection in hostile environments. Fighter pilots are trained for this and they are taught survival skills where they land. There are some psychological aspects of this too.

The K-36 is rated for inverted ejection down to 100ft. Clearly, the rockets do more than enable a zero-zero ejection. The thrusters do indeed change the flightpath of the seat, so No is the wrong answer.

I know fighter pilots are trained to survive in enemy territory, but the army/navy would always want to avoid that situation. In today's technological world, where there are portable jets which help people fly, I am sure the military are working on something.

That "something" is unmanned aerial vehicles -- i.e. drones.

Back when I went through USAF pilot training I remember being told our ejection seats had gyroscopes in the seat which would rotate the seat upright following an ejection.  Unable to find a source confirming this now, though.
Although a self-righting seat wouldn't be effective against enemy weapons, it might help against mother nature.  And it's pretty cool technology.  The violence of an ejection is probably a more important threat than a later threat from an enemy (the enemy can't kill a crew member who doesn't survive the ejection).  Ejections from unusual attitudes do happen.


text:
Can an outside observer tell if an aircraft is flying through turbulence?
As a passenger, when I look out of the window, I can see the wings shaking. What would another plane (not in turbulence) see if they were observing the same plane (in turbulence)?

If you can see the wings flexing from inside the aircraft then you would see that from outside too :-) And the whole aircraft would move up and down in the air slightly. Are you asking how far (how many feet/metres) an aircraft can be pushed up and down by turbulence? You might also want to check out the previous questions on turbulence, especially this one and this one.

The wings act as a structural dampener to disturbances such as turbulence, similar to the suspension on a car. As they flex, they absorb the sudden energy changes. The aerodynamic forces also take time to accelerate the large mass of the airplane. Because of these effects, most of turbulence will be too small to see from an outside observer's point of view.
However, in addition to the wing flexing, the aircraft will also roll, pitch, and yaw in response to the turbulence. The pilots or the autopilot, as well as the natural stability tendencies of the aircraft, will help the aircraft return to its previous state.
Here is a video recorded from the X-Plane flight simulator program of a 737 in turbulence. The wing flex and change in flight attitude can be seen.

The wings are flexing because there is a change in lift.
The wings are more aerodynamic than the body of the aircraft. I.e. the wings are more "responsive", they respond to changed aerodynamic forces more rapidly.
As a result of the change in lift in the wings, the body (being connected to the wings) follow the change.

Therefore, to an observer who is immune to turbulence, the wings would flex first, followed by the aircraft as a whole. If the turbulence is causing the wings to flex up and down repeatedly (e.g. imagine sine wave), the body of the aircraft can be observed lagging behind the wings and smaller in amplitude.

Let's suppose you stand on an unstable bar stool. It shakes a little when you move, but you feel a lot of movement. An observer standing close by will see the stool and you shake a little bit (a few inches at most). If that observer moves a 100 feet away, he might not notice the shaking at all.
When the airplane you are on is in turbulence1, you will feel a lot of shaking. The airplane is shaking, but to an outside observer which is close, it will feel as much as you feel it. Again, if the observer is moved a mile away, the shaking will not appear as significant as your experience.

1 Minor turbulence. In case of severe turbulence, it will be more evident to an observer.


text:
Was this the intended flight path?
I have verified numerous approaches from the NW to PVR airport and they all look like this, basically approaching on the left side of the mountain then taking a right towards the airport:

But then noticed 1 flight whose path is different:

Do you think that was the intended flight path or they suddenly realized they need to be further to the left ?

That last one looks much more like a DME arc, often implemented as part of an approach; unfortunately, I don't have any Puerto Vallarta approach plates handy with which to confirm or refute this.

Puerto Vallarta (MMPR) has a VOR/DME on the field (PVR), which is used in seven different approaches (that I can find) depending on where you're coming from and which runway you're landing on:

VOR DME 1 Rwy 04 (Teardrop)
VOR DME 2 Rwy 04 (DME arc from NW)
VOR DME 3 Rwy 04 (DME arc from SE)
VOR DME 1 Rwy 22 (Teardrop)
VOR DME 2 Rwy 22 (DME arc from NW)
VOR DME 3 Rwy 22 (DME arc from SE)
VOR Rwy 04 (Procedure Turn)

Your abnormal flight seems to exactly match the VOR DME 2 Rwy 22 coming from BLASK on airway V1/J141. So yes, that was the intended flight path.
What's interesting is that your normal flights don't match any of the procedures. I suspect that ATC is vectoring those planes to final, which saves the time, fuel and effort of flying a DME arc, teardrop or procedure turn.
If ATC can do that, though, why did that one flight do the DME arc as charted? It's possible ATC was too busy to vector at that time, or their radar was down, or there was a radio problem, or the pilot requested the full procedure to maintain proficiency, or something else. Without a recording of the ATC frequency, it's hard to say.

license: Creative Commons - Attribution Share-Alike - https://creativecommons.org/licenses/by-sa/4.0/


text:
What is a jet engine's fuel flow at constant IAS?
Here is my assumption:

In straight and level flight of a jet turbine engine:
One IAS = One drag = One thrust = One fuel flow

Therefore, the fuel flow will be approximately the same for a given IAS, regardless of altitude, temperature, or RPM necessary to maintain this IAS.
Is that correct? If not, where is the fallacy?
EDIT : Here is a screenshot from ATPL course material which supports this idea and led me to ask the question as it seems doubtful to me. Propulsive efficiency vs RPM at high altitude at the end is not clear as well.

This is very much dependant on altitude, air pressure, temperature... Thinner air creates less drag, has less oxygen, etc. Not too mention that "IAS" changes with altitude all by itself...

Related: Why do jet engines get better fuel efficiency at high altitudes?

@Ron Beyer. It's true that IAS increases (in TAS terms) with altitude, but the OP presumes a fixed IAS...

@Ron Beyer as IAS is fixed, thinner air will not create less drag. IAS reflects dynamic pressure, so if maintained constant, dynamic pressure will be the same, and so will drag. It's true though that air gets thinner with altitude, but it is compensated by the increased TAS (more air, so overall dynamic pressure is constant). This part by itself is counter intuitive

@Bianfable Thanks for the link, I'm familiar with this topic. Engines get better fuel efficiency at high altitudes because for a same IAS, so a same fuel flow from my point of view, the TAS is much greater: therefore the consumption per unit of distance is much less

@ArnaudPROST But the higher TAS for same IAS is due to the lower air density at higher altitudes. The better fuel efficiency (at least in part) is due to the lower temperatures, so you should get lower fuel flow for the same IAS at higher altitudes.

@Bianfable this is the point, but I'm not sure. If the temperature is lower (density higher), the TAS necessary to get this IAS will be lower. But the drag will be the same, and so will, in level flight, the thrust. And for me in a jet engine, if you set a thrust, the fuel flow is set as well.

The most obvious difference is due to the temperature of the air.
Both turbine and piston engines are heat engines. They work by converting thermal power into mechanical power. The theoretical absolute maximum efficiency you can achieve is called the Carnot efficiency, $$\eta = 1-\dfrac{T_C}{T_H}$$
This is the efficiency of an ideal engine using the Carnot cycle, that works by transferring heat from a hot reservoir with temperature $T_H$ to a cold reservoir with temperature $T_C$. A typical jet engine is approximated by the Brayton cycle, and a piston engine the Otto or Diesel cycle, but neither can ever by more efficient than the efficiency noted above. The efficiency of a Brayton cycle is $$\eta = 1-\dfrac{T_C}{T_E}$$ with $T_E$ the EGT.
When flying higher, the temperature of the cold reservoir (the atmosphere) drops lower. You can see that the maximum efficiency of the engine will also increase (even if $T_H$ or $T_E$ respectively drop simultaneously with $T_C$). This means that, even if the required power would stay constant for constant thrust, the fuel flow changes, because a single unit of energy from a drop of fuel can be converted into more mechanical power.

thanks, I'm almost convinced. Here are my doubts: 1/ even though the Carnot cycle efficiency is very much dependent on temperature difference between hot and cold reservoir, is it the case for the Brayton cycle ? The maximum (Carnot efficiency) could vary with this temperature while the Brayton, staying below, would not be affected.  2/ Why wouldn't the hot reservoir just become colder (EGT lower) ? 3/ If the temperature is colder, density higher therefore TAS will be lower to fly at same IAS. Doesn't this compensate the extra energy from cold air ?

1/ I quoted Carnot as to avoid the inevitable question "what if we used another cycle". I'll add it to the answer. 2/ If the temperature rise is approximately constant (which makes sense because $c_p$ is approximately constant as the atmosphere is approximately an ideal gas) you still get a rise in efficiency $1-\frac{T_C}{T_h}<1-\frac{T_C-\Delta T}{T_H-\Delta T}$. 3/ That effect is already accounted for if we only look at the IAS.

Could you edit your answer to elaborate on 3/ ? It's not clear to me, and the whole difficulty of the problem. If we had fixed TAS, the efficiency difference would be sufficient to explain different fuel flows. But we fixed IAS, so even though more energy is produced out of a unit of fuel in colder air, in this colder air TAS will be lower to maintain the IAS. So less work will be needed, and less fuel.

@ArnaudPROST You're thinking the wrong way around. IAS is dynamic pressure, so what the aircraft actually 'feels' - it doesn't matter if that's due to temperature or altitude that the density drops. But after establishing that constant IAS leads to roughly constant thrust, you can then wonder how efficient thrust is generated. Density doesn't really come into the whole thermodynamic efficiency picture.

I agree, but thermodynamic is not the only parameter influencing the efficiency of thrust generation, TAS is one as well. Thrust is approximately: $T=\dot{m_{air}}(V_{jet}-TAS)$ Thermo tells you how hard it is to accelerate the airflow to $V_{jet}$, but for a same thrust, you will need more or less of it depending on TAS

Your answer focus on thermal efficiency, which is only part of the overall propulsive efficiency which determines fuel flow $\eta=\eta_{cycle}\eta_{prop}$ with $\eta_{prop}=\frac{2}{1+\frac{V_{jet}}{TAS}}$. From "Gas turbine theory" by Cohen and Rogers, overall propulsive efficiency is proportional to $\frac{TAS}{SFC}$. My assumption is that on iso-IAS lines, this value is constant

@ArnaudPROST Perhaps you are right along adiabatic lines, but it is possible to have the same IAS and TAS with a different temperature. So while your statement may be true by approximation, there's no fundamental law that says it should be.

The fallacy is

One thrust = One fuel flow

The thrust-specific fuel consumption (TSFC) of a jet engine (the mass of fuel consumed per unit of thrust) isn't constant, so that equality is violated.  TSFC will vary with all the usual factors:  ambient temperature and pressure, airspeed, etc.

Could you perhaps elaborate why this is the case?

Honestly, come to think of it, you can have multiple level flights at the same IAS with different AoAs or configurations, and thus different drags, too, so that part's also trivially fallacious, but the question also seemed to be about engines instead.  Where it's also trivially fallacious because there's a whole concept devoted to the idea that one thrust doesn't equal one fuel flow.

@Erinn Anne this is true about configurations, but the question of course considers a constant configuration. Could you elaborate on different drags and AoAs for the same IAS on level flight ? It sounds wrong to me, from ATPL at least. For your answer, I also disagree: the difficulty is that ambient temperature and pressure changes, as well as airspeed, are included in IAS parameter (from density). IAS being fixed, it does not matter if one is changed, the others will compensate to maintain the same value of IAS and it could therefore be the same from the engine point of view.

On configuration:  you could deploy some drag, like flaps or spoilers, add an equal amount of thrust, and now you're at the same IAS (because thrust - drag is still 0 and haven't changed altitude).  So one IAS absolutely doesn't correspond to one drag (hence what we call the "drag coefficient", which changes with configuration, or with airframe...)
On thrust:  TSFC is an empirically observed fact.  Look at the tables in the wikipedia article I linked.  You can thought experiment at it all you like, but the reality is what it is.

@Erin Anne: configuration is constant. When you said "you can have multiple level flights at the same IAS with different AoAs OR configuration" it's false.  For a level flight and an IAS you have only one AoA. Your reference to TSFC is not clear enough, and not linked to the statements of the question. Please elaborate your reasoning: "reality is what it is" does sound like an authority argument which does not help to understand underlying phenomena

@Erin Anne: in addition, TSFC is not equal to fuel flow, so the logic of the argument in your answer is violated.

Per my answer above, TSFC is equal to fuel flow per unit thrust, (not just simply fuel flow) and it empirically changes in response to factors like altitude (one of the factors you specifically name "regardless of" in the question), so it invalidates "one thrust = one fuel flow."  It is literally the number that tells you how much fuel you will burn at a given thrust.

I understand your point, but I do think it is incomplete, because it does not take into account the iso-IAS hypothesis. You look at it on first order only. TSFC of course changes with temperature, but also with airspeed, RPM, pressure, as you said. IAS links all those parameters together (you act on the engine so RPM to maintain the IAS, airspeed changes with temperature for the same IAS, etc...). Since then, it is absolutely possible than on iso-IAS lines, TSFC is constant. At least, you answer does not prove the opposite


text:
What are the requirements to convert an FAA certificate to EASA/JAA?
If a pilot holding a US certificate wanted to convert it to an equivalent EASA (JAA?) certificate, what training, flight experience, and exams are necessary to obtain equivalency?
Does the process differ for private, instrument, commercial, ATP, and instructor ratings?

We had three guys converting FAA licenses to EASA here in 2013. The CAA decided on a specific conversion program for each candidate based on type of license, experience, etc. All three had to take all 14 ATPL theory exams. Actual flight training varied around 10 to 15 hours in a SEP and about 3 hours in a MEP.

As @Pondlife states, this will depend on each member state's implementation of the EASA rules but I would expect that the implementations would be similar -- otherwise what is the point of EASA?  (That would be a good question except it would no doubt be closed as being too "opinion based".)
I've added a link to the relevant UK documentation and marked this as a Community Wiki answer so that hopefully others can expand on this for other states and perhaps add personal experiences.
UK
CAP 804 Section 4, Part Q deals with "Validation and Conversion of Non-EASA licences issued by States other than the UK".

@lnafziger managed to copy this answer just before you deleted your duplicate question!

Haha, good!  I delayed deleting it hoping that you would get it!

I have no personal experience of this question but it is included in the EASA Flight Standard FAQ. The key point seems to me to be this:

The competent authority of the Member State to which an applicant
  applies will determine the conversion requirements, which can be
  reduced on the basis of a recommendation from an approved training
  organisation.
Therefore, the national aviation authority of the Member State where
  an applicant resides or wishes to work should be contacted for further
  information concerning the applicable acceptance requirements.

In other words, the exact requirements are determined locally by the EASA member state you live in and/or want to fly in. Perhaps someone who's actually been through the process could give a better answer.


text:
Is there a limit of how close a runway can be to the water?
Somtimes planes can either come up short or overrun a runway, ending up in the water.
Is there some limit for how close a runway can be to the water? Does it factor into the runway safety area that is required to be clear of obstacles (i.e., is water an obstacle)? Is there any requirement for displaced thresholds near water?
I am asking primarily about the US, but information about any other jurisdictions is welcome.

If SFO is any indication, it can be awful close: http://www.richp.com/pics/aerial-05-20-01/sfo1.jpg

Check out KCOI: https://img.airnav.com/ap/01869.jpg?v=JFCCQ4 -- a small GA airport for sure, but certainly close to water.

Doesn't get any closer to water than London City (LCY)

Boston Logan and, to a lesser extent, JFK are also very close. Outside the US, London City and Hong Kong come to mind.

@greener Beat me to LCY by four seconds! :-) But there are plenty closer than LCY. The overrun (is that the right word?) at the south-east end of 15R at Boston Logan is built out over Boston Harbour, for example.

@DavidRicherby Genoa, Tokyo Haneda, Lampedusa(?) and Gibraltar as well

Sydney (SYD) can be also a good example.

OK, so are there actually any airports that aren't right next to water? ;-)

In addition to JFK, consider LaGuardia -- which is also close to the water vertically; major storms have sometimes put the runway under water and shut down the airport. (I had a flight cancelled for that reason once.) And I seem to remember that JFK can handle seaplanes.

@DavidRicherby: Yes, there are. On some of them overrun would be much worse than on the near-water ones. And often there is some water anyway

Saba's runway is both very short (396m) and surrounded by water and cliffs.

Let's raise the bar a bit: the runway of Madeira Airport is above water.

Don't forget Kansai in Japan, it is surrounded by water on all sides and built on an artificial island.  Overruns, short landings or loss of directional control (to one side) will put you in the water.

Check out Logan (Boston) [Google Maps 42.363300,-71.008250]
Every runway at this major hub has at least one end in the water.

Isn't there some beach (in Scandinavia?) where scheduled flights take off and land (only at low tide)?

Barra Airport, Scotland the runway is on the beach.

@keshlam - it was LGA that was originally a flying boat (not seaplane) base, and the still-in-use Marine Air Terminal is a reminder of those years https://en.wikipedia.org/wiki/Boeing_314_Clipper

Could swear that I've gone past a marine air terminal at JFK, when getting lost in the parking lots... but I certainly could be misremembering.

Of course sometime the water IS the runaway ...

@DavidRicherby: Not just the overrun.  Almost all of Logan is on top of what used to be harbour.  (Which is why part of eastern Logan is actually in Winthrop rather than Boston; the water boundary between the two was drawn long before the Logan landfilling, and part of the land reclaimed for the airport ended up lying east of that boundary, and, hence, being part of Winthrop.)

@ANeves: Aren't parts of both runways at la Guardia also built out above the water?

yet another one: Wellington Airport is very close to water at both ends. It looks like the southern end was extended over the road (creating a tunnel) to add some safety area.

FAA Advisory Circular (AC) 150/5300-13A covers "standards and recommendations for airport design." It does not include any restrictions on proximity to bodies of water, but it does make the following recommendation in paragraph 319(a):

It is recommended that the entire RSA and RPZ be accessible to rescue and fire-fighting vehicles such that no part of the RSA or RPZ is more than 330 feet (100 m) from either an all-weather road or a paved operational surface. Where an airport is adjacent to a body of water where access by rescue personnel from airport property is desirable, it is recommended that boat launch ramps with appropriate access roads be provided.

The actual FARs (14 CFR 139, Certification and Operations: Land Airports) also do not state a limit. The only reference to bodies of water of any kind is in §139.325 (Airport emergency plan) paragraph (e):

The plan required by this section
  shall contain provisions, to the extent
  practicable, for the rescue of aircraft
  accident victims from significant bodies
  of water or marsh lands adjacent to
  the airport which are crossed by the
  approach and departure flight paths of
  air carriers. A body of water or marsh
  land is significant if the area exceeds
  one-quarter square mile and cannot be
  traversed by conventional land rescue
  vehicles. To the extent practicable, the
  plan shall provide for rescue vehicles
  with a combined capacity for handling
  the maximum number of persons that
  can be carried on board the largest air
  carrier aircraft that the airport reasonably
  can be expected to serve.

So, bottom line: there is no limit, but airports are required to take water into due consideration when developing emergency plans, and the FAA looks at each airport and plan on a case-by-case basis during airport certification.
As for the rest of the world, I have no authoritative information; but as others have posted, it certainly seems like the situation is the same, there being numerous examples of airports worldwide built on natural or reclaimed land very close to water.

There is no such limit indeed. Princess Juliana International Airport in Sint Maarten is possibly the most iconic example of airport located near a water body, because of the beach over which airliners fly when approaching RWY 09. As other mentioned, they are numerous other examples of airports that may be even closer to a water body (i.e. without a beach in between).

image source

What is so close to the water is not the runway, but a blastpad. The runway at Saint Marteen airport is almost 190m inland.

Water is not an obstacle. There are plenty of examples around the world where runways are built on purpose-reclaimed land and the ocean starts just a handful of meters from the end of the runway.
For some random examples, check Nice-Côte d'Azur or Gibraltar or St. Maarten.

...Or La Guardia

Runway 28L and 28R at KSFO, as well as 19L and 19R, have their thresholds right next to the San Francisco Bay.
If you recall, in July 2013, Asiana Airlines Flight 214 landed short due to pilot error.
In November 1968, Japan Airlines Flight 2 completely missed the runway and landed in San Francisco Bay.

Not going by Barra Airport -

Barra Airport is famous for its beauty - boasting beaches, machair and hills - and all in a small island; Barra is a special place to visit - especially memorable if you fly in because of the beach landing strip. Washed by the tide twice a day, Traigh Mhor beach is reputed to be the only beach runway in the world to handle scheduled airline services.

See Barra Airport website

According to Wikipedia, the runway at Barra is also popular with cockle pickers; they have to keep an eye on the windsock to see if planes might be landing.

The answer to the question is yes, there are limits. I'll expand for ICAO regulated airports, but this is also true for FAA ones.
ICAO defines an area surrounding a runway, called runway strip, which purpose is to:
a) to reduce the risk of damage to aircraft running off a runway; and
b) to protect aircraft flying over it during take-off or landing operations.

A water runway strip would not reduce the risk of damage to aircraft running off a runway. It would in fact increase it, so a runway strip cannot be made of water.
The size of the runway strip depends on the type of runway, but it ranges 150 m to 0m (zero)  in width, and it must extend 30 to 60 m beyond each end of the runway (or the end of the stopways, if there are any).
So, the runway strip contains at least the runway (and its stopways) and extends beyond it on its ends, and sometimes (depending on the type of runway) it also extends to the sides of the runway, and it defines the limits to how close a runway can be to water.
Going even further, ICAO defines a Runway End Safety Area (RESA) for some types of runways which further increases the minimum distance of water to the runway.
Airports such as Sint Marteen, where the runway is almost 190m from the water, are just examples of how the runway strip and the RESA are applied as safety measures against aircraft running off a runway.

Here is Albert Whitted Airport, a GA facility in St. Petersburg, FL, which has actually juts out into Tampa Bay.


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How is/was RFID bag handling introduced in a baggage handling system tailored to barcodes?
I have just seen Richard Hammond's Engineering Connections about Hong Kong Chek Lap Kok (built 1998) and afterwards, Big, Bigger, Biggest about London Heathrow Terminal 5 (built 2008).
Quote from Eng. Conn.:

Hong Kong was one of the first airports to replace bar codes with RFID tags to track bags

Quote from BBB:

Even such an advanced [baggage handling] system [as the new one in T5] still relies on good old-fashioned Bar Codescanners.

Which made me think: Bar codes are, as far as I know, standardized by IATA, so luggage can go end-to-end through a multitude of airports on one bar code ("check-through"), but I guess when the first airport switched over to RFID, there wasn't one for RFID, so there's a bit of a hen-egg problem. How did Hong Kong airport assure that all luggage passing through it had an RFID tag?

RFID + Barcode tags are used for luggage departing from HK airports only.

Old-style Barcode tags are not fully replaced by RFID one until late 2008.

By printing them along with bar codes. RFID mode became operational in HK in 2005. Basically, the airport replaced the barcode only baggage tags with RFID embedded tags:

Hong Kong International Airport (HKIA) today announced that all of its check-in counters can now print integrated radio frequency identification (RFID) baggage tags. The new tags, which combine an embedded RFID chip with a barcode, are replacing barcode-only baggage tags on an airline-by-airline basis.

Basically, the system reads both the RFID tags and barcodes.

But this does not work for luggage that does come in from other airports, right? So they didn't replace the bar codes, they still have barcode scanners everywhere for luggage from outside?

@Alexander Correct. That's why they still have barcode readers for baggage coming from elsewhere